Chief operating officer, Tim Cook, will take over Jobs' responsibilities

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Apple CEO Steve Jobs is shown in this combination of file photographs. Top row, from left, shows Jobs in July 2000, November 2003 and September 2005. Bottom row, from left, shows September 2006, January 2007 and September 2008. Jobs said Wednesday he will take a medical leave of absence until the end of June because his health problems are "more complex" than he had thought.

Apple Inc. co-founder and Chief Executive Steve Jobs said Wednesday he is taking a medical leave until the end of June — just a week after the cancer survivor tried to assure investors and employees his recent weight loss was caused by an easily treatable hormone deficiency.

Apple’s stock plunged 7 percent.

Jobs, 53, said in a letter last week that he would remain at Apple’s helm despite the hormone problem, and that he had already begun a “relatively simple and straightforward” treatment. But in an e-mail to employees Wednesday, Jobs backtracked.

“During the past week I have learned that my health-related issues are more complex than I originally thought,” he wrote.

Apple’s shares have surged and crashed over the last year in step with rumors or news about the CEO’s health and his gaunt appearance. While the top executive’s health is an issue for investors in any company, at Apple the level of concern reaches fever pitch because Jobs has a hand in everything from ideas for new products to the way they’re marketed. Investors fear that without Jobs, Apple will not be able to sustain its growth of the last decade, which has seen Apple branch out from its Mac computers into the iPod and the iPhone.

Last week, Jobs said his disclosure of his hormone problem was “more than I wanted to say, and all that I am going to say” about his health. It came on the eve of Macworld, the biggest Apple trade show of the year, and Jobs said he wanted everyone to relax and enjoy the show.

Even so, the limited amount of medical information in that announcement did little to soothe Wall Street’s nerves, and in interviews last week analysts predicted that the health watch would continue.

Apple’s history of keeping information about Jobs’ health under wraps is only fueling the speculation. The company waited until after Jobs underwent surgery in 2004 to treat a very rare form of pancreatic cancer — an islet cell neuroendocrine tumor — before alerting investors. That type of cancer is easily cured if diagnosed early, unlike the deadlier and more common adenocarcinoma.

And last summer, Apple insisted Jobs’ weight loss was due to a common bug, even as The New York Times cited anonymous sources who said Jobs had undergone “a surgical procedure” to address the problem.

Apple’s chief operating officer, Tim Cook, will take over Jobs’ responsibilities while he is on leave, though Jobs said he plans to remain involved in major strategic decisions while he is out.

Cook is seen as one of Jobs’ most likely successors, along with Apple’s top marketing executive, Philip Schiller.